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Video: Worlds collide

LOS ANGELES — The Spitzer Space Telescope's examination of hundreds of stars has found evidence that the time it takes to form an Earth-sized planet may last hundreds of millions of years — much longer than previously thought.

The telescope revealed dust rings around nearby stars that couldn't have survived long unless violent collisions between gigantic chunks of rock were replenishing them, scientists said during a news conference Monday at NASA's Jet Propulsion Laboratory in Pasadena.

Scientists have long believed that planets are formed when the dust in a disclike formation around a young star begins to clump. Some of the clumps eventually grow to the size of mountains, and these smash into each other, where some of the matter is absorbed to create a larger embryonic planetoid.

These, in turn, collide with each other, creating more dust and rocky chunks that keep smashing into each other. In some theories, this brutal stage of planet growth lasts perhaps 10 million years or so — an eye blink in astronomical terms. Then there is a long, steady, quieter cleanup period in which the unused dust dissipates.

Scientists found something interesting, however, when NASA's Spitzer Space Telescope turned its infrared eye on the stars.

Of 266 stars observed, 71 had dusty discs, but instead of having faded, some discs were big and bright even though their stars were 100 million to 200 million years old. The dust, detected via its heat signature because it is warmed by the star, would have dissipated in that time unless big, recent collisions were replenishing it, researchers said.

The collisions appear to involve a planet colliding with something 10 to 200 miles (16 to 320 kilometers) in diameter.

"Now, we will have to rewrite the textbooks ... the disks can last much longer than previously thought and can be replenished," said Jonathan Gardner, Spitzer program scientist in Washington.

The latest findings provide more clues to how motes of dust become a world. The process may be how our own solar system was created and also could be another indicator that Earth-sized planets are common in the universe, researchers said.

There is a "clear link" between formation of planets in our solar system and those around other stars, said Scott Kenyon, a senior astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Mass., who studies the formation of planetary systems.

"When we look out into space, we see our own history," Kenyon said.

The fact that other stars show the dust rings "probably means planets are forming around all these stars," he added.

Spitzer was launched Aug. 25, 2003, as the fourth member of NASA's Great Observatories program, following the Hubble Space Telescope, the Chandra X-Ray Observatory and the Compton Gamma Ray Observatory, which is no longer in orbit.